There are two things you should consider:
1. The shock sensor will not work for the HDD in optibay slot. Hence, if you use a non-SSD drive it won't be protected from bumps.
2. Optibay has SATA2 (3Gb/s) instead of SATA3 (6Gb/s). If you decide to use SSD with optibay slot it might not be worth buying the fastest SSD money can get (there are one or two out there that can saturate SATA3) but instead get a speedy mid-range SSD.
Also, one more point to note:
256GB SSDs are *faster* than 128GB SSDs (at least the current gen). This is simply because 256GB consists of more physical memory chips and hence there are more channels than can be written to/read from at the same time. If 128GB was made of 4 memory chips, 256GB would be made of 8. There is a big speed gain there; so I guess it should be better to get a decent 256GB SSD instead of most expensive 128GB.